Posted by: Rob Viens | December 12, 2012

Amphipods, Isopods, I-pods and Gamera

For the last week the Beagle continued to travel southward and most of Darwin’s entries were short and focused on the weather. Overall it seemed to be pretty decent conditions and the Beagle made good progress:

“During these two delightful days we have been gliding onwards; but at a very slow pace.” (Dec 5/6)

“Fine, light weather.” (Dec 7/8)

“From the high irregular swell, there must have been bad weather to the South, so that we are lucky in escaping it.” (Dec 9)

“A strong breeze; At noon we were a little to the South of Port Desire.” (Dec 10)

As they approached the 12th of December the weather started to change, and with it Darwin’s stomach took a turn for the worse:

“The Barometer had given good warning of a change of weather: it is the anniversary of our first attempt to get out of the English channel, & as on that day we were met by a heavy breeze from the SW.— With me the association was perfect though not very satisfactory, between the two days: my stomach plainly declared it was of terrestrial origin & did not like the sea.” (Dec 11)

“It continued to blow fresh & in the middle of the day suddenly freshened into the heaviest squall I have ever seen. Luckily it gave us good notice, so that every thing was furled & the ship put before the wind; it is always interesting to watch the progress of a squall; the black cloud with its rising arch which gives passage to the wind; then the line of white breakers, which steadily approaches till the ship heels over & the squall is heard whistling through the rigging.— The climate during the few last days has undergone a complete change.— The Temp. varies from 45° to 50°, & the air has the bracing feel of an English winter day: But the most curious thing is to see the hammocks piped down at ½ after seven & the sun some way above the horizon.— it is a spectacle we have not beheld for the last 15 months.” (Dec 12)

Only one additional comment suggests that Darwin was also having some fun with his home-made plankton net (see Science is Everywhere). In the entry from Dec 5/6 he notes:

“I have been employed in examining some small Crustacea; most of which are not only of new genera, but very extraordinary ones.” (Dec 5/6)

Crustacea are a subphylum of the Amphopoda (along with insects, spiders, millipedes, etc.) that includes shrimp, crabs, lobsters, barnicles, as well as their smaller cousins the copepods, ostracods and amphipods. (Most are aquatic and marine, however there are several terrestrial orders, including isopods such as woodlice and “rolly pollies”.)

Darwin’s Zoological Notebook suggests that much of what he was finding were smaller swimming Crustacea such as amphipods or the larva of larger orders such as the Mantis Shrimp (for more on these fellas see Darwin’s Shrimp).

One of specimen Darwin describes in some detail, he refers to as “Cyclops”. Footnotes suggest that this is the copepod from the Order Calanoida, a major component of the oceanic food web. This little planktonic Calanoids form a major source of food for many fish and baleen whales. Without them many fisheries would be in big trouble.

Diaptomus – a type of Calanoid copepod (from Wikipedia commons)

Diatomus

Another species Darwin describes in an amphipod of the suborder Hyperiidea (shown below in an image from Wikipedia Commons).

Hyperiidea

Below is a part of Darwin’s description of these little crustaceans:

“Tail formed of 6 pieces or 3 pair.— central in shape are flat, spear-shaped, pointed, sending off a small pointed external plate.— they have two articulations.— the central pair are seated more a posteriori than the others, but are of equal length in themselves.— the external pair are narrower than the others.— These organs when expanded form a fan & are most essential to the animal in swimming.— 3 pair of swimming plates, these are bifid.— divisions equal with many joints.— Body with 7 segments, & lateral plates by the base of legs, coloured with stars of purple: Eyes exceedingly large; forming the whole anterior part of head.— transparent, containing an oblong opake part.— of fine purple colour.” (Zoological Notebook)

Isopods and amphipods have some similarities – they are typically small, lack a carapace, and have specialized limbs (including some with respiratory organs). But what makes them different?  Well if pressed to tell them apart, here are some things to look for:

  • Isopods are vertically flattened (similar to a flounder) while amphipods are flattened side-ways (similar to a sunfish).
  • Typically the limbs on an isopod come in two varieties – one type attached to the abdomen and the other to the thorax.  On the other hand, amphipods can have several different types of limbs that each have different functions.
  • Lastly, if you look closely you will see that amphipods typically have an arched body structure compared to their flatter cousins the isopods.

Amphipod species Gammarus roeselii (from Wikipedia Commons):

Gammarus

Both amphipods and isopods are old organisms, with representatives going back well over 250 million years ago to the Paleozoic Era.

OK, I lied in my title – no i-pods today, but imagine what Darwin’s notes would have looked like if he had an i-pad… (RJV)

PS – So what I really want to know is … Are the two crustacean groups – Gammaridea and Godzilliidae – named after the movie monsters Gamera and Godzilla?  If so, I find that to be very cool. Especially considering these crustaceans are only about 1 cm long. Are there other crustaceans with such names? Watch out Tokyo – or at least Tokyo’s tide pools!

Gamera – one of the ultimate movie monsters – on a Dark Horse comic book (from Wikipedia Commons)

Gamera

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